We use radio frequency (RF) communication in our own daily activities (or call your relatives, or send text messages to friends, or even reading this blog article through mobile devices). There are many signals in the air to spread rapidly, however, where most such signals are generated? Most RF communications are derived from a cellular tower or a wireless base station, such as the base station tower shown in Figure 1.
Figure 1: Mobile Base Station
There are many components in these base stations, so a complete summary will become a doctoral paper! But the author will not go to the face, but focus on a component that is critical to any base station: power amplifier (PA).
As you may have guess, the purpose of PA is to enhance the low power RF signal into a high power RF signal (driven into the base station transmitter).
Before injected with any RF signal, a DC (DC) voltage (Vgate) is applied to the gate of the PA and adjust the voltage until the desired drain current flows through the PA. This current is typically referred to as a quiescent current - a current flowing when there is no RF input. The static current value can be selected to fit the final application (including the modulation system and device run level). Figure 2 is a simplified schematic diagram of a typical PA setting.
Figure 2: Simplified PA Schematic
To better understand how the gate voltage and static current affect RF (AC (AC)) performance, you can use a metal oxide semiconductor field effect transistor (MOSFET) model to replace PA, resulting in the following expression:
(MOSFET in saturated state) (1)
(2)
Figure 3 is a graphical representation of two equations. A small RF input signal is driven to superimpose to the DC gate voltage to produce an AC drain current ΔIDS. The AC current oscillates around the static current value IDSQ (see Figure 3a). You can find the corresponding AC drain voltage ΔVDs using the MOSFET transistor I-V curve and load line analysis (see Figure 3b). Equation 2 can be further simplified into equation 3 when determining the relationship between the AC drain voltage and the AC drain current.
(3)
In Figure 3a, the results GM are calculated using a transconductive slope, and you can further summarize the expression as:
(4)
Therefore, the voltage gain ΔVds / ΔVgate of the amplifier is interpreted as -RS • GM, which can be converted to:
-RS • 2 • IDSQ / (Vgate - Vth)
The above expression primarily clarifies the gain of the configuration and the static current IDSQ directly proportionally. In addition, IDSQ can be selected to ensure that the output voltage swing will not be limited due to saturation; this is why you should choose an IDSQ value before RF running, which requires a predetermined DC gate voltage.
Figure 3: (a) MOSFET VGATE and IDS graph
(B) MOSFET common source load line analysis
Traditionally, all PA bias systems use a discrete solution, and some such solutions are as simple as the potentiometer (variable resistor divider) on the PA gate. The newer method utilizes the accuracy and digital interface of the Precision Mold Converter (DAC) and / or Digital Potentiometer. Different PA techniques such as transverse diffusion MOSFET (LDMOS), gallium gallium gallium nitride (GaAs) require different levels of gate voltages for devices operating. For example, GaN and GaaS require a negative bias system, while LDMOS requires a positive voltage suitable for use in devices. For this reason, many PA offset solutions are now listed as part of DAC with bipolar ranges.
The nonlinearity shown by PA is mainly dependent on the temperature. These nonlinearities can significantly affect performance because they lead to unpredictable device behavior.
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